1. Field of the Invention
This invention relates to a failure determination system for a continuously variable transmission of a belt type which has a belt stretched between two pulleys of a variable pulley width type, and performs speed varying operation by varying the pulley widths of the pulleys by oil pressure supplied thereto via control valves, and a failure determination system for a start clutch disposed between a transmission and drive wheels of a vehicle and having the engaging force thereof controlled by oil pressure supplied thereto via a control valve, and more particularly to determination of failure of these control valves.
2. Description of the Related Art
Conventionally, a failure determination system for a continuously variable transmission has been disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. S60-157553 (pages 2-12 and FIG. 13). The continuously variable transmission is installed on a vehicle, and comprised of a drive pulley of a variable pulley width type to which torque from an engine is input, a driven pulley of a variable pulley width type connected to the drive pulley via a belt, a hydraulic circuit including three solenoid valves for controlling oil pressure supplied to the drive pulley and the driven pulley, and a controller connected to the three solenoid valves via three electric drive circuits. In the continuously variable transmission, command signals from the controller are input to the solenoid valves via the electric drive circuits, respectively, to control the solenoid valves, thereby supplying oil pressure from the hydraulic circuit to the drive pulley and the driven pulley to change the pulley widths of the pulleys, whereby the speed varying operation of the continuously variable transmission is carried out.
Further, the failure determination system includes two rotational speed sensors for detecting the respective rotational speeds of the drive pulley and the driven pulley. In the failure determination system, the inputs and outputs of the electric drive circuits are compared with each other during travel of the vehicle, whereby it is determined whether or not a disconnection or a short circuit has occurred in the solenoids of two of the solenoid valves for supplying oil pressure to the drive pulley (step SR13 in FIG. 13 of the publication). Further, during travel of the vehicle, the actual transmission ratio of the continuously variable transmission is calculated based on signals delivered from the two rotational speed sensors, and determination of whether the speed varying operation of the continuously variable transmission is faulty is executed based on the difference between the detected actual transmission ratio and a target transmission ratio (steps SR16 to SR19 in FIG. 13 of the publication).
According to the conventional failure determination system, the determination of whether or not a disconnection or a short circuit has occurred in the solenoids of the solenoid valves is performed separately from the determination of faulty speed varying operation of the continuously variable transmission, and hence it is possible to discriminate failure of the solenoids and failure of the hydraulic circuit of the continuously variable transmission from each other. However, each solenoid valve is comprised of a combination of a solenoid and hydraulic circuit components, such as a valve element, not only the solenoid but also the hydraulic circuit-components can become faulty. For example, there are cases in which the valve element is fixed in the open-state or the closed-state, e.g. due to a foreign matter contained in oil. Further, in the hydraulic circuit of the continuously variable transmission, a foreign matter in oil or a small amount of oil leakage at a location other than the three solenoid valves can cause shortage of oil pressure supplied to the pulleys. However, in the above failure determination system, only the failure determination of the solenoids of the solenoid valves is carried out, so that it is impossible to discriminate failure of a hydraulic circuit component of the solenoid valves from failure of components of the hydraulic circuit other than the solenoid valves. As a result, even when only a hydraulic circuit component of the solenoid valves is faulty, the faulty component cannot be identified, but it is only possible to determine the failure of the hydraulic circuit as a whole. To locate a faulty portion of the hydraulic circuit, it is necessary to check the entire hydraulic circuit, which results in degraded maintainability of the continuously variable transmission. Particularly when locating of a faulty portion is impossible, it is necessary to replace the whole hydraulic circuit.
A determination system for a start clutch arrange in the drive system of a vehicle has been conventionally proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. H02-176265 (pages 4-5 and FIG. 6). This failure determination system is for determining failure of a lockup clutch of a torque converter, and comprised of an engine speed sensor for detecting an engine speed, a TM rotational speed sensor for detecting a TM rotational speed input to a transmission, and a microcomputer to which all of these sensors are connected. Further, a solenoid valve is connected to the microcomputer, for controlling oil pressure supplied via a hydraulic circuit to the lockup clutch according to a command signal from the microcomputer, whereby the engaging force of the lockup clutch is controlled.
In the proposed failure determination system, when the command signal is input to the solenoid valve from the microcomputer, thereby controlling the lockup clutch to the engaged state, failure of a lockup system including the solenoid valve and the hydraulic circuit is determined based on results of comparison between the engine speed and the TM rotational speed, in other words, based on the results of comparison between the input-side rotational speed of the lockup clutch and the output-side rotational speed of the same. More specifically, when a slip ratio as a quotient of division of the difference between the engine speed and the TM rotational speed by the engine speed is more than a predetermined value B (3%), it is determined that the lockup system is faulty. Further, in this failure determination system, the command signal input to the solenoid of the solenoid valve is compared with a monitor signal monitoring the solenoid from a monitor circuit of a drive circuit, whereby occurrence of a disconnection or a short circuit in the solenoid is determined.
Further, a start clutch arrange between an automatic transmission of the drive system of a vehicle and drive wheels of the same has been conventionally proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2002-39352 (pages 4-5 and FIG. 1). In this start clutch, an electric control valve controls oil pressure supplied from an oil pressure pump to the start clutch whereby the engaging force of the start clutch is controlled. Further, the automatic transmission is implemented by a continuously variable transmission of a belt type having a belt stretched around two variable pulleys.
When the conventional failure determination system disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H02-176265 is applied to the start clutch disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2002-39352, there arises the following problems: According to the conventional failure determination system disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H02-176265, the determination of whether or not a disconnection or a short circuit has occurred in the solenoid of the solenoid valve is performed separately from the determination of failure of the lockup system, and hence it is possible to discriminate failure of the solenoid and failure of the lockup system from each other. However, according to this failure determination system, only the failure determination of the solenoids is carried out, so that it is impossible to discriminate failure of a hydraulic circuit component of the solenoid valves from failure of components of the hydraulic circuit other than the solenoid valves, for the same reason described hereinabove as to the conventional failure determination system for a continuously variable transmission. As a result, even when only a hydraulic circuit component of the solenoid valve is faulty, the faulty component cannot be identified, but it is only possible to determine the failure of the hydraulic circuit as a whole. To locate a faulty portion of the hydraulic circuit, it is necessary to check the entire hydraulic circuit, which results in degraded maintainability of the start clutch. Particularly when locating of a faulty portion is impossible, it is necessary to replace the whole hydraulic circuit.